Abstract

The Popeye domain–containing 1 (POPDC1) gene encodes a plasma membrane–localized cAMP-binding protein that is abundantly expressed in striated muscle. In animal models, POPDC1 is an essential regulator of structure and function of cardiac and skeletal muscle; however, POPDC1 mutations have not been associated with human cardiac and muscular diseases. Here, we have described a homozygous missense variant (c.602C>T, p.S201F) in POPDC1, identified by whole-exome sequencing, in a family of 4 with cardiac arrhythmia and limb-girdle muscular dystrophy (LGMD). This allele was absent in known databases and segregated with the pathological phenotype in this family. We did not find the allele in a further screen of 104 patients with a similar phenotype, suggesting this mutation to be family specific. Compared with WT protein, POPDC1S201F displayed a 50% reduction in cAMP affinity, and in skeletal muscle from patients, both POPDC1S201F and WT POPDC2 displayed impaired membrane trafficking. Forced expression of POPDC1S201F in a murine cardiac muscle cell line (HL-1) increased hyperpolarization and upstroke velocity of the action potential. In zebrafish, expression of the homologous mutation (popdc1S191F) caused heart and skeletal muscle phenotypes that resembled those observed in patients. Our study therefore identifies POPDC1 as a disease gene causing a very rare autosomal recessive cardiac arrhythmia and LGMD, expanding the genetic causes of this heterogeneous group of inherited rare diseases.

Figure 1

(A) Pedigree of a family homozygous for the POPDC1S201F mutation. The index patient (PTIII-2) and his brother (PTIII-1) suffered from AV block and the grandfather (PTI-1) from LGMD and AV block. (B) TEM analysis of skeletal muscle of PTI-1 revealed the presence of plasma membrane discontinuities (arrows) and subsarcolemmal and intermyofibrillar vacuoles (asterisks). C and D show higher magnifications of the boxed areas in B. Scale bars: 2 μm (B); 1 μm (C and D). Data depicted are representative of results derived from a single biopsy. (E) Holter ECG of the index patient (PTIII-2) displaying an episode of a paroxysmal AV block. (F) Electropherogram of the Sanger sequencing of genomic DNA isolated from family members. (G) Sequence alignment of part of the Popeye domain demonstrating sequence conservation. Many residues are identical (turquoise) or similar (green); however, S201 (yellow) is ultraconserved. (H) 3D model of the Popeye domain. Only the phosphate-binding cassette (PBC) is shown. Two hydrogen bonds are formed by S201, but these are lost when the F201 (turquoise) mutant residue is present, possibly affecting the ligand-binding affinity of the Popeye domain.